1. Field of the Invention
The present invention relates to an objective lens holding device and an optical pickup apparatus.
2. Description of the Related Art
As conventional recording media for information recording/reproduction, there are optical discs such as a CD (Compact Disc) using infrared laser light, a DVD (Digital Versatile Disc) using red laser light, etc. Recently proposed are an optical disc (HD DVD (High Definition DVD) using blue-violet laser light and a Blu-ray Disc (registered trademark), both capable of higher density recording than the conventional optical discs. To perform information recording/reproduction on those optical discs, there exists e.g., an optical pickup apparatus having a single lens holder mounted with an objective lens for focusing laser light for the conventional optical discs and with an objective lens for focusing blue-violet laser light for the Blu-ray Disc, etc. The disposition of the two objective lenses relative to an optical disc can be e.g., as shown in FIGS. 9A and 9B, such that they are disposed in the direction of a radius (hereinafter, referred to as radial direction) of the optical disc (FIG. 9A) or such that they are disposed in the direction of a tangent (hereinafter, referred to as tangential direction) of tracks spirally formed on an information recording layer of the optical disc (FIG. 9B)
In the information recording/reproduction on the optical disc, a tracking control is executed to cause a laser light to follow a track to be a target of information recording/reproduction (hereinafter, referred to as information recording/reproduction target track). The tracking control employs in general a differential push-pull method, a three-beam method, etc., wherein three laser lights (0th-order light, ±1st-order diffracted lights) are used, which are obtained by diffracting laser lights by e.g., a diffraction grating. The differential push-pull method, etc., wherein the three laser lights are used, make it possible to provide better tracking control than the conventional push-pull method, etc. by applying +1st-order diffracted light and −1st-order diffracted light line-symmetrically with respect to the tangential direction of the information recording/reproduction target track.
In the case of disposing two objective lenses 101 and 102 in the radial direction (FIG. 9A), in any case when emitting 0th-order light and ±1st-order diffracted lights from the objective lenses 101 and 102, +1st-order diffracted light and −1st-order diffracted light can be applied line-symmetrically with respect to the tangential direction of the information recording/reproduction target track, enabling the tracking control based on the differential push-pull method, etc. On the other hand, in the case of disposing the objective lenses in the tangential direction (FIG. 9B), when emitting 0th-order light and ±1st-order diffracted lights from the objective lens 103, +1st-order diffracted light and −1st-order diffracted light can be applied line-symmetrically with respect to the tangential direction of the information recording/reproduction target track. However, when emitting 0th-order light and ±1st-order diffracted lights from the objective lens 104, +1st-order diffracted light and the −1st-order diffracted light are applied line-asymmetrically with respect to the information recording/reproduction target track.
This may result in fluctuations in amplitude of tracking error signal corresponding to the reflected lights of 0th-order light and ±1st-order diffracted lights, making infeasible the tracking control based on the differential push-pull method, etc. Therefore, in the case of disposing the objective lenses in the tangential direction, another push-pull method, etc. based on 0th-order reflected light is employed, which may be inferior to the differential push-pull method, etc. in tracking control.
In this manner, when mounting two objective lenses on a single lens holder in the optical pickup apparatus, it is desirable to choose the disposition in the radial direction, whereby better tracking control based on the differential push-pull method, etc. can be executed.
Such a technique is disclosed in International Publication WO98/02874.
However, in the case of disposing the two objective lenses 101 and 102 in the radial direction, e.g., when the radially outside objective lens 102 focuses laser light on a track of an information recording layer 112 of an optical disc 107, the radially inside objective lens 101 is positioned more radially inside than in the case of disposing the lenses in the tangential direction or in the case of mounting only a single objective lens in the lens holder. When the radially outside objective lens 102 focuses laser light on, e.g., a track of a more radially inside area (hereinafter, referred to as innermost circumference of the information recording layer) than a read-in area (or PCA (Power Calibration Area)) of the information recording layer 112 of the optical disc 107, the radially inside objective lens 101 is positioned most inside in the radial direction. In the more radially inside area than the read-in area, etc., there is recorded information indicating, e.g., that the optical disc 107 is Light Scribe support. In the case of simply using a lens holder 105 having specifications of no need to move to a more radially inside area than a read-in area (or PCA), since proper restrictions are not imposed on the shape of the lens holder 105 or on the objective lenses 101 and 102, the edge portion 110 of the radially inside objective lens 101 may butt against a turntable 109 when moving to the innermost circumference of the information recording layer 112 as shown in FIGS. 10 and 11. FIG. 10 depicts a movement of the lens holder 105 to the innermost circumference of the information recording layer 112, wherein the position of the periphery of the turntable 109 in the radial direction is set to be zero and the distance between the turntable 109 and the edge portion 110 of the objective lens 101 is denoted as M. Note that the distance M on the plus (+) side denotes the distance M when the edge portion 110 is apart from the periphery of the turntable 109 and that the distance M on the minus (−) side (inclusive of zero, the same applies hereinafter) denotes the distance M when the edge portion 110 is assumed to move further inside of the zero position. FIG. 11 is a plan view of FIG. 10.
Even though the edge portion 110 of the objective lens 101 does not butt against the turntable 109 (i.e., the distance M is on the plus(+) side) when moving to the innermost circumference of the information recording layer 112, there might be a possibility of the edge portion 110 of the objective lens 101 butting against the turntable 109 in the following case. That is, the distance M at that time becomes less than the amount of radial movement of the lens holder 105 as a result of tracking control for causing laser light from the radially outside objective lens 102 to follow the innermost circumference of the information recording layer 112. If the edge portion 110 of the objective lens 101 butts against the turntable 109, then it may become difficult to focus laser light on the innermost circumference of the information recording layer 112, and to record/reproduce information on/from the innermost circumference of the information recording layer 112. Further, if the objective lenses 101 and 102 tilt together with the lens holder 105 as a result of butting, then it may become difficult to focus laser light on tracks of the overall information recording layer 112, and to record/reproduce information on/from the optical disc 107.
It can be considered that one approach to solve the problems is reducing the diameter of the objective lenses 101 and 102. However, a reduction in diameter of the objective lenses 101 and 102 results in reducing a effective diameter for focusing laser light, whereupon if an actuator (not shown) mounted with the objective lenses 101 and 102 shifts in the radial direction to follow the disc eccentricity, etc., then there may arise, e.g., a reduction in quantity of light of laser light focused on the optical disc 107 or a degradation in amplitude of a signal (reproduced signal, etc.) corresponding to the reflected light of laser light, which may induce another problem.
Therefore the object of the present invention is to provide an objective lens holding device and an optical pickup apparatus, enabling a radially outside objective lens to focus laser light on an innermost circumference of an information recording layer of an optical disc without a radially inside objective lens butting against a turntable.